While Adele caused a storm last week with a rude gesture, the "V" sign offends me on an almost daily basis. Until I belatedly discovered the apparently well-known fact that parmesan is made using calf rennet and is unsuitable for vegetarians, I merrily ate platefuls of pesto-drenched pasta with the hard cheese shaved liberally across it, safe in the knowledge that no restaurant would say something was suitable for vegetarians when it wasn't. How wrong I was.

A trawl of veggie web forums reveals heated debates on the subject going back years (Word of Mouth readers brought the subject up again recently in the comments on this post). The message clearly wasn't getting through, though, because in 2010 the Vegetarian Society launched its Say Cheese campaign to help make restaurateurs aware of their error when shaving heaps of the hard stuff over food which they then credit as suitable for vegetarians: diners were encouraged by the charity to leave cards in offending restaurants explaining the mistake.

The next few years could spell more for restaurateurs than disappointed diners, as the terms "vegetarian" and "vegan" will soon have legal status. UK Food Standards Agency labelling guidelines were adopted in principle by the European Union in 2010, and following a five year period for compliance civil suits may be brought against anyone misusing the terms from 2015. Just as a maker of parmesan can bring action against anyone outside the region using the p-word, so could offended diners against clueless chefs. Restaurants, manufacturers and publishers will really have to know their cheeses.

Anyone familiar with the blank look on a waiter's face when asked if their pesto is vegetarian ("yes, pesto is just basil and pine nuts", "and parmesan?", "oh yes, and parmesan, but that's just cheese") will know that sometimes it is easier to avoid such dishes. Or, where parmesan's presence is not stated on the menu, hastily push any offending shavings to the side of your plate before they start to melt.

As it is, the law states that cheese is only allowed to be labelled Parmigiano Reggiano or parmesan (the widely used French name) if it meets a number of criteria, including being made using calf rennet. It's nothing new; parmesan has had Protected Designation of Origin status since 1996.

It was a trip to Pizza Express which eventually enlightened me. The vegetable-laden Fiorentina pizza sounded right up my street – spinach, free range egg and glorious shavings of parmesan – but the "V" was notable by its absence; the menu stated that vegetarian diners should ask for it without the cheese. Eureka! Despite spending an inordinate amount of time scouring the backs of sweet packets for gelatine, I'd completely overlooked the fact that parmesan, by its nature, is not vegetarian. But then, it seems, so have many chefs.

Hugh Fearnley-Whittingstall caused controversy among clued-up veggies last year by including parmesan in his first vegetarian cookbook, River Cottage Veg Every Day!. Publisher Bloomsbury are on another print run now (due to its success, I should add, rather than a few irate comments on the River Cottage forums), but apparently this time without the errors.

Rose Elliot buoyed my former ignorance with her reliance on parmesan in several books I used to see as veggie bibles, while Delia Smith has been called for using it in Delia's Vegetarian Collection and online recipes. One commenter on her forum makes the point that, since the cheese is cited as non-vegetarian elsewhere on the site, surely this should be stated in the recipe too. Vegetarians are left to make their own substitutions and presumably cross their fingers that potential dinner party hosts will too.

In 2010, a guest at Raymond Blanc's Le Manoir stormed out when a waiter insisted that a parmesan topped canapé was suitable for vegetarians. On realising his error, Blanc contacted the diner to apologise, writing on his blog: "we discovered that for years we have been giving cheese containing animal rennet to our vegetarian guests ... Various forms of vegetarianism are the norm, and as a good restaurateur it is our duty to adapt and respond to these new needs and to our guests' rising expectations." Amen!